Stepping away from the Defiler for a moment, I thought this was worth mentioning.
If you’ve got a 99 Cent Store or something similar near you, go check out their Halloween section. I just found a four pack of plastic shot glasses shaped like skulls. Yeah, plastic shot glasses aren’t that great, but the real way to look at this is it's a pack of four 1 3/4” skulls for a dollar.
Imperial scenery. Chaos scenery. Super-heavy vehicles. Doesn’t matter what color they are once you prime them and paint them like granite, marble, or steel. If you can’t think of something in Warhammer 40K you might want to build that would benefit from a skull bigger than a Space Marine... well...
For a buck or two, there are worse things for a scratch builder to have in his or her bitz box.